Well, as I'm sure you realize, I think it varies a lot, especially dependent on the person's upbringing, surroundings, lifestyle, age when they lost their virginity, etc.
For example, I'm 22 and have had 9 partners.
My boyfriend is 28 and has had 3.
My ex-roommate is now about 26 I think and has had over 100.
I did a quick google search and found an article from MSNBC on the topic: New survey tells how much sex we’re having.
It's pretty interesting. It surveyed around 1600 people between 20 and 59, and claims "The median number of lifetime female sexual partners for men was seven; the median number of male partners for women was four."
I've read this article before. Of course, in my opinion, seven partners for men and four for women seems very low but we must remember that many people - girls especially - are pressured to wait until marriage that would leave their number at 1. And so, some people's numbers skew something like the "average" to something that seems very low.
Also, I do think it's probable that 29% of men and 9% of women have had 15 or more partners. I think, most people find this to be a rather high number to be honest (not me personally). Also, I've heard that in some cities something like 7 partners is completely average but in New York due to high population/busy lifestyle/ease of casual hookups the average is more like 30.
My number is less than 10 (I'm 25) but if I were to find myself single again (I'm very happy currently and feel completely comfortable doing the long-term thing right now....) I am almost certain that my number would exceed 10 by the time I'm 32.
At one time in my life, I was on a sexual journey and wanted something like 150 partners. I even told my boyfriend this the night I met him! But now, I'm completely happy keeping my number where it's at because I've learned in regards to the number of partners that more partners doesn't necessarily mean they will enrich my knowledge of my sexual self. I'm learning way more in this exclusive relationship than with all my other partners combined.